X-ray imaging technology provides a non-invasive technique for visualizing the internal structure of an object of interest by exposing the object to high energy electromagnetic radiation (i.e., X-rays). X-rays emitted from a radiation source pass through the object and are absorbed at varying levels by the internal structures of the object. As a result, X-ray radiation exiting the object is attenuated according to the various absorption characteristics of the materials which the X-rays encounter.
The absorption characteristics of the object of interest may be captured by placing the object between a high energy electromagnetic radiation source and an image recording medium. As radiation from the source passes through the object, the radiation impinges on the image recording medium with an intensity related to the radiation attenuation caused by the different absorption characteristics of the object. The impinging radiation causes a change in the image recording medium that is proportional to the radiation intensity, thereby storing information about the internal structure of the object. The image recording medium may then be processed to recover the stored information by, for instance, converting it into digital form. Common types of image recording media include sheet film, phosphor media, and the like.
Phosphor plate technology has emerged as a valuable image recording media for computed radiography (CR). When electromagnetic radiation, such as X-ray radiation, impinges on a phosphor plate, the radiation interacts with the phosphor lattice of the plate. The phosphors in the plate store energy proportional to the intensity of the impinging radiation. This energy can later be released by scanning the plate with a laser to excite the phosphors in the plate (i.e., by causing the phosphors to fluoresce). The excited phosphors release radiation that can be detected, quantified and stored as values representing pixels in an image.
As discussed above, radiation penetrating an object being imaged is attenuated according to the various absorption characteristics of the object, e.g., Z-number, density, and the like. In addition, radiation penetrating an object will also undergo a characteristic attenuation due to the thickness of the attenuating material. The thickness of a material refers to a dimension of the material generally in the direction penetrating radiation is propagating. However, this characteristic attenuation is not a linear function of material thickness. Rather, as the thickness of the material increases, the intensity of penetrating rays of radiation will decay exponentially.
FIG. 1 illustrates the exponential relationship between the thickness of intervening material and the attenuation of the penetrating radiation. Object 100 is composed of a material having a uniform density p and a linearly increasing thickness along the z-axis. For example, object 100 may be a piece of aluminum having a ramp shape in the z-direction. Object 100 is illustrated as comprising five layers, each layer adding an incremental thickness z0. Object 100 is exposed to radiation 105 which penetrates object 100 in a direction substantially parallel to the z-axis, as illustrated by exemplary rays 105a-105f. The thickness of the arrows schematically denotes the relative intensity of the radiation of the respective ray.
As shown, exemplary rays 105a-105f enter object 100 at substantially the same intensity (e.g., at the intensity of radiation provided by the radiation source). After the radiation penetrates object 100, it impinges upon phosphor plate 120, which absorbs the radiation and stores energy proportional to the intensity of the impinging rays. As shown by exemplary rays 115a-115f, the increasing thickness of object 100 will attenuate the radiation by increasing amounts. For example, ray 115a undergoes relatively small attenuation as the thickness of object 100 near y=0 is essentially negligible, while ray 115b experiences a relatively large attenuation due to the increased material thickness. As discussed above, this characteristic attenuation does not increase linearly with thickness, but rather the intensity of radiation 105 decreases exponentially with thickness. That is, the change in intensity of radiation 105 between each incremental thickness z0 will start out relatively large (e.g., the large change in intensity between layer 100a and 100b) and will become relatively small to negligible (e.g., the small change in intensity between layers 100d and 100e) as a function of increasing thickness.
The attenuation of radiation through a solid material can be described generally according to the characteristic attenuation function:I=I0e−μ(z)  (1)where I is the intensity of rays having penetrated the material through a thickness z, I0 is the intensity of the radiation emitted from the radiation source before penetrating the material, and μ is the linear absorption coefficient for the material. The linear absorption coefficient μ incorporates various absorption and scattering effects including Thompson scattering, Compton scattering, photoelectric (PE) absorption, pair production, photodisintegration, and the like, and is different for different materials. At radiation power levels typical of diagnostic imaging, the PE absorption is a main contributor to radiation attenuation, and depends, in part, on the density of the material through which the radiation penetrates.
FIG. 2 illustrates a plot 200 including a curve 210 showing the ratio of emitted radiation intensity (I0) to penetrating radiation intensity (I) as a function of thickness z, caused by material having a linear absorption coefficient μ, for example, object 100 in FIG. 1. The ratio ranges from a value of 1 at a thickness of zero (i.e., zero attenuation) and asymptotically approaches zero as a function of increasing thickness (i.e., approaches infinite attenuation).
As a result of the characteristic exponential attenuation, radiation impinging on the phosphor plate will have an exponential bias to material thickness. For relatively small thicknesses, the contrast resolution will be relatively high, while for relatively large thicknesses, the contrast resolution will be relatively small. Stated differently, the change in radiation attenuation will be much more significant for first increments of additional material thickness than for later increments. For example, radiation penetrating object 100 will undergo a greater range of ray attenuation while penetrating layer 100a than it will penetrating 100b. When the radiation penetrates to layer 100f, changes in radiation per incremental thicknesses approaches negligible.
Accordingly, radiation impinging on the phosphor plate will have an exponential distortion as a function of material thickness. When the energy stored in the phosphor plate (i.e., the latent image) is released, the intensity of the released energy may not appropriately reflect the density of the subject matter through which it penetrated, and in turn, the resulting digital image may not be an accurate depiction of the internal structure of the object being imaged, since the contrast resolution will vary across the dynamic range.
Many conventional processing techniques have attempted to compensate for this phenomenon by inverting the effects of the exponential attenuation. For example, rearranging equation (1) and taking the natural logarithm of both sides results in the expression:ln(I/I0)=−μ(z)  (2)which results in a more intuitive linear relationship between attenuation and material thickness z. Accordingly, conventional systems have applied a logarithmic amplifier to detection signals generated during scanning of a phosphor plate to compensate for the characteristic exponential attenuation. The amplified detection signals will appear as if each incremental thickness in material causes the same amount of attenuation on the penetrating radiation, regardless of whether the increment is the first portion of a material or the last portion of the material. That is, logarithmic amplification adjusts values of a detection signal substantially to correspond to a constant change in radiation intensity as a function of material thickness.
FIG. 3 schematically illustrates a scanning process 300 of a conventional computer radiography imaging process. Radiation 305 (e.g., X-ray radiation) is emitted from a radiation source to expose an object being imaged. Radiation 305 may be emitted over some desired area, for example, a region of a patient undergoing a diagnostic procedure. Accordingly, radiation 305 will have an intensity distribution I0(x,y) in the x-y plane and will propagate generally in the z-direction to penetrate the object. As discussed above, the radiation penetrating the object will undergo attenuation that can be generally modeled by the expression in equation (1) as shown in attenuation block 310. Radiation 305′ exiting the object will have an intensity distribution I(x,y) that depends on the absorption characteristics of the object being imaged, and will carry some distortion due to the exponential attenuation characteristic. Radiation 305′ may then impinge on an image recording medium, such as a phosphor plate 320, thus storing the intensity distribution I(x,y) as a latent image in the phosphors of the plate.
The energy stored in phosphor plate 320 may then be released by scanning the phosphor plate with stimulating radiation, for example, a laser beam. The laser beam causes the phosphors in the plate to fluoresce and release stimulated radiation in proportion to the amount of energy stored in the phosphors. Scanning apparatus adapted to release a latent image from an image recording medium are known in the art, for example, the scanning apparatus described in U.S. Pat. No. 6,624,438 (Koren), which is herein incorporated by reference in its entirety. An exposed phosphor plate may be scanned, by means known to those skilled in the art, by providing the laser beam such that it impinges on the phosphor plate in a regular path, traversing the surface of the phosphor plate to release energy stored at various locations along the path in a substantially serial manner.
The phosphor plate then releases energy region by region as the laser traverses in time along the scan path. The energy from each region may ultimately correspond to a pixel in the resulting image. Accordingly, the energy released by the phosphor plate includes radiation having an intensity distribution as a function of time. For example, phosphor plate 320 may release energy by emitting stimulated radiation 315 having an intensity distribution I′(t). Since the scan path is generally planned, the timing of the released energy encodes the location from which the energy was released.
Radiation 315 may be sensed by a detector responsive to stimulated radiation to generate a detection signal indicative of the intensity of the radiation. For example, a photomultiplier tube (e.g., PMT 350 in FIG. 3) may be arranged proximate the phosphor plate such that at least some of radiation 315 emitted from the phosphor plate impinges on the photosensitive surface of PMT 350. In response, PMT 350 generates an electrical signal having a magnitude indicative of the intensity of the impinging radiation. In FIG. 3, PMT 350 generates an electrical signal 325 that varies in magnitude according to the intensity distribution of radiation 315 (e.g., according to I′(t)).
However, the distortion caused by the characteristic exponential attenuation of the object being imaged is propagated through the process and may therefore be carried by electrical signal 325. That is, electrical signal 325 may be a distorted description of the density characteristics of the object being imaged. To compensate, conventional systems have provided electrical signal 325 to logarithmic amplifier 360 to essentially invert the effects of the characteristic exponential attenuation. Logarithmically amplified signal 325′ may then be provided to an analog-to-digital converter (ADC) 370 to convert the electrical signal into a digital signal 335 to form a digital image 390. The resulting image may then be transmitted, further processed and/or displayed such that the internal structures of the object may be viewed.
Issues associated with image read-out are described in U.S. Pat. No. 5,357,118 (Fukuoka), U.S. Pat. No. 5,278,754 (Arakawa), and U.S. Pat. No. 5,535,289 (Ito), all incorporated herein by reference.